The emergence of SARS-CoV-2 led to pandemic spread of coronavirus disease 2019 (COVID-19), manifesting with respiratory symptoms and multi-organ dysfunction. Detailed characterization of virus-neutralizing antibodies and target epitopes is needed to understand COVID-19 pathophysiology and guide immunization strategies. Among 598 human monoclonal antibodies (mAbs) from ten COVID-19 patients, we identified 40 strongly neutralizing mAbs. The most potent mAb CV07-209 neutralized authentic SARS-CoV-2 with IC 50 of 3.1 ng/ml. Crystal structures of two mAbs in complex with the SARS-CoV-2 receptor-binding domain at 2.55 and 2.70 Å revealed a direct block of ACE2 attachment. Interestingly, some of the near-germline SARS-CoV-2 neutralizing mAbs reacted with mammalian self-antigens. Prophylactic and therapeutic application of CV07-209 protected hamsters from SARS-CoV-2 infection, weight loss and lung pathology. Our results show that non-self-reactive virus-neutralizing mAbs elicited during SARS-CoV-2 infection are a promising therapeutic strategy.

In late 2019, an outbreak of a severe respiratory disease caused by an emerging coronavirus, SARS-CoV-2, resulted in high morbidity and mortality in infected humans. Complete understanding of COVID-19, the multi-faceted disease caused by SARS-CoV-2, requires suitable small animal models, as does the development and evaluation of vaccines and antivirals. Since age-dependent differences of COVID-19 were identified in humans, we compared the course of SARS-CoV-2 infection in young and aged Syrian hamsters. We show that virus replication in the upper and lower respiratory tract was independent of the age of the animals. However, older hamsters exhibited more pronounced and consistent weight loss. In situ hybridization in the lungs identified viral RNA in bronchial epithelium, alveolar epithelial cells type I and II, and macrophages. Histopathology revealed clear age-dependent differences, with young hamsters launching earlier and stronger immune cell influx than aged hamsters. The latter developed conspicuous alveolar and perivascular edema, indicating vascular leakage. In contrast, we observed rapid lung recovery at day 14 after infection only in young hamsters. We propose that comparative assessment in young versus aged hamsters of SARS-CoV-2 vaccines and treatments may yield valuable information, as this small-animal model appears to mirror age-dependent differences in human patients.

In COVID-19, immune responses are key in determining disease severity. However, cellular mechanisms at the onset of inflammatory lung injury in SARS-CoV-2 infection, particularly involving endothelial cells, remain ill-defined. Using Syrian hamsters as a model for moderate COVID-19, we conduct a detailed longitudinal analysis of systemic and pulmonary cellular responses, and corroborate it with datasets from COVID-19 patients. Monocyte-derived macrophages in lungs exert the earliest and strongest transcriptional response to infection, including induction of pro-inflammatory genes, while epithelial cells show weak alterations. Without evidence for productive infection, endothelial cells react, depending on cell subtypes, by strong and early expression of anti-viral, pro-inflammatory, and T cell recruiting genes. Recruitment of cytotoxic T cells as well as emergence of IgM antibodies precede viral clearance at day 5 post infection. Investigating SARS-CoV-2 infected Syrian hamsters thus identifies cell type-specific effector functions, providing detailed insights into pathomechanisms of COVID-19 and informing therapeutic strategies.

Highlights d Three closely related dwarf hamster species are susceptible to SARS-CoV-2 infection d The course and outcome of infection vary dramatically between species d Apparent differences in disease progression are independent of the ACE-2 sequence d The Roborovski dwarf hamster is a valuable addition to current SARS-CoV-2 models

22In late 2019, an outbreak of a severe respiratory disease caused by an emerging 23 coronavirus, SARS-CoV-2, resulted in high morbidity and mortality in infected humans 1 . 24Complete understanding of COVID-19, the multi-faceted disease caused by SARS-CoV-25 2, requires suitable small animal models, as does the development and evaluation of 26 vaccines and antivirals 2 . Because age-dependent differences of COVID-19 were identified 27 in humans 3 , we compared the course of SARS-CoV-2 infection in young and aged Syrian 28 hamsters. We show that virus replication in the upper and lower respiratory tract was 29 independent of the age of the animals. However, older hamsters exhibited more 30 pronounced and consistent weight loss. In situ hybridization in the lungs identified viral 31 RNA in bronchial epithelium, alveolar epithelial cells type I and II, and macrophages. 32Histopathology revealed clear age-dependent differences, with young hamsters launching 33 earlier and stronger immune cell influx than aged hamsters. The latter developed 34 conspicuous alveolar and perivascular edema, indicating vascular leakage. In contrast, 35we observed rapid lung recovery at day 14 after infection only in young hamsters. We 36propose that comparative assessment in young versus aged hamsters of SARS-CoV-2 37 vaccines and treatments may yield valuable information as this small-animal model 38 appears to mirror age-dependent differences in human patients.